Environ. Sci. Technol. 2000, 34, 4088-4094
Relating Liquid Fuel and Headspace Vapor Composition for California Reformulated Gasoline Samples Containing Ethanol ROBERT A. HARLEY* AND SHANNON C. COULTER-BURKE Department of Civil and Environmental Engineering, University of California, Berkeley, California 94720-1710 T. S. YEUNG Monitoring and Laboratory Division, California Air Resources Board, 9528 Telstar Avenue, El Monte, California 91731
Changes in gasoline formulation will be required as use of methyl tert-butyl ether (MTBE) is phased out. Changes in evaporative emissions of volatile organic compounds (VOCs) may be of concern in cases where ethanol is added to gasoline to replace MTBE. Regular, mid-, and premium grade gasoline samples containing ethanol were collected in the San Francisco Bay area during May 1999. The compositions of the liquid fuel samples and their headspace vapors were measured. Ethanol contents ranged from 3.25 to 9.65 wt % in the liquid fuel samples. Four compounds (nbutane, n-pentane, 2-methylbutane, and ethanol) together accounted for >50% of the total headspace vapor mass. The partial pressure of ethanol in headspace vapors increased only modestly, despite a 3-fold increase in ethanol in the liquid samples of premium vs regular grade gasoline. Reactivity with respect to ozone formation of the liquid fuel samples was dominated by aromatics, whereas the reactivity of headspace vapors was dominated by alkanes and cycloalkanes. The olefin and sulfur contents of the liquid fuel samples were unusually low. Equilibrium headspace vapor composition was predicted using measured liquid fuel composition, pure liquid vapor pressures of each fuel constituent, and activity coefficients derived from equilibrium P, T, x, and y measurements for ethanol in solution with gasoline hydrocarbons. Weight fractions of the 13 most abundant species in gasoline headspace vapors were predicted to within (15% of measured values, except for n-butane which was overpredicted by 22%. Alternate predictions of headspace vapor composition using the universal functional-group activity coefficient (UNIFAC) model were generally less accurate, especially for ethanol.
Introduction The properties of gasoline have been altered in recent years to reduce motor vehicle emissions of carbon monoxide, * Corresponding author. Department of Civil and Environmental Engineering, University of California, Berkeley, California 947201710. Phone: (510) 643-9168; fax (510) 642-7483; e-mail: harley@ ce.berkeley.edu. 4088
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photochemical smog precursors, and toxic organic air pollutants such as benzene. Changes have been made to sulfur, olefin, and aromatic contents, and to distillation properties of gasoline. Controversy has surrounded another major fuel change: the addition of oxygenates such as methyl tert-butyl ether (MTBE) and ethanol to gasoline (1,2). Presently there is no requirement that a specific oxygenate be added to gasoline. However, use of MTBE in gasoline will be phased out in California by the end of 2002 due in part to concerns about surface water and groundwater contamination. Likewise, the U. S. EPA intends to reduce significantly the use of MTBE in gasoline nationwide (3). These decisions will lead to greater dependence on ethanol-gasoline blends and/or gasoline formulations that do not contain oxygenated compounds. Various approaches to using ethanol as a motor vehicle fuel have been considered, ranging from 5 to 10 vol % ethanol blended in some gasoline sold in the United States, to 22 vol % ethanol in much of the gasoline sold in Brazil (4), to fuels containing as much as 95 vol % ethanol (E95). There is concern about increased evaporative emissions of volatile organic compounds (VOCs) when ethanol is blended with gasoline, because such blends tend to have higher Reid vapor pressure (RVP) than equivalent MTBE-blended fuel (2). It is necessary to know the chemical composition of VOC emissions to assess their ozone-forming potential. Chemicalcomposition profiles (also referred to as speciation profiles or source fingerprints) also are needed for receptor-modeling studies (5,6) that seek to determine source contributions to ambient VOC concentrations. Kirchstetter et al. (7) have shown that gasoline reformulation has led to major changes in exhaust and evaporative VOC emission speciation, so many existing speciation profiles reported in the literature are not applicable to present-day conditions. Evaporative emissions occur by a wide variety of mechanisms, including fuel spillage and vapor displacement during refueling, venting of fuel tank vapors as ambient temperature changes, fuel evaporation from the engine compartment of parked vehicles due to residual engine heat, liquid leaks in vehicle fuel systems, and so on. The composition of evaporative emissions depends on the mechanism by which VOCs are emitted. Liquid fuel composition may be used to speciate emissions due to fuel leaks and spills, whereas the composition of gasoline headspace vapors is more appropriate to speciate VOC emissions due to vapor displacement. For wellfunctioning modern vehicles with low emission rates, vaporliquid equilibrium theory cannot explain fully the measured composition of evaporative VOC emissions (8). However, liquid fuel and equilibrium headspace vapor composition profiles are more likely to describe evaporative emissions from high-emitting vehicles with liquid and/or vapor leaks in the fuel system. Unfortunately, the chemical compositions (resolved to the level of individual organic compounds) of liquid gasoline and gasoline headspace vapors are rarely measured. In the cases where measurements are made, often only liquid fuel samples are analyzed, so methods to predict headspace vapor composition are needed. Predictions made using measured liquid fuel composition and ideal solution theory have been shown to agree with measured headspace vapor composition for gasoline-hydrocarbon mixtures and gasoline-MTBE blends (7,9). However, the assumption of ideal behavior is not appropriate for ethanol-gasoline mixtures. In solution with nonpolar gasoline hydrocarbons, hydrogen bonding between ethanol molecules is reduced, and the contribution 10.1021/es0009875 CCC: $19.00
2000 American Chemical Society Published on Web 08/31/2000
of ethanol to gasoline vapor pressure can exceed that predicted using ideal solution theory by factors of 2-8 or more, as discussed below. The objectives of this study are to (1) measure liquid fuel and headspace vapor compositions for gasoline samples containing ethanol that meet California phase 2 reformulated gasoline program requirements, (2) assess the reactivity of liquid fuel and headspace vapor samples with respect to ozone formation, and (3) predict headspace vapor composition from measured liquid fuel composition, accounting for non-ideal-solution behavior of ethanol-gasoline blends.
Methods Fuel Sampling and Analysis. Regular, mid-, and premium grade gasoline samples expected to contain ethanol were collected at a fuel tanker-truck loading facility in the San Francisco Bay area by California Air Resources Board (CARB) personnel during May 1999. About 750 mL of each gasoline sample was collected in a 1-L aluminum container. Fuel samples were sent to the CARB Monitoring and Laboratory Division in El Monte, CA for analysis. Detailed hydrocarbon analysis of fuel samples was performed using a gas chromatograph (GC) with a 100 m × 0.2 mm i.d. Petrocol DH-100 capillary column, a 2-3 m × 0.2 mm i.d. DB-5 precolumn, and a flame ionization detector (FID) (10). Results for oxygenates such as ethanol and MTBE are only approximate by this method. Oxygenates in liquid fuel samples were quantified separately using another GC equipped with a polar precolumn (560 mm × 1.6 mm o.d., stainless steel, 20% 1,2,3-tris-2cyanoethoxypropane on 80/100 Chromosorb P/AW), a nonpolar analytical column (30 m × 0.53 mm i.d., 3.0 µm film thickness, DB-1), a column switching valve, and an FID (11). The precolumn retained the oxygenates and the heavier hydrocarbons, while allowing the faster-eluting light hydrocarbons to pass through. The precolumn was then backflushed onto the analytical column, which separated the individual C1-C4 alcohols, ethers, and tert-pentanol from the remaining heavier hydrocarbons. Headspace Vapor Sampling and Analysis. In the laboratory, gasoline samples were stored in their aluminum containers in a refrigerator at approximately 0 °C. Each gasoline sample was removed from the refrigerator, and approximately 60 mL of the sample was transferred to a 60mL amber glass bottle. The bottles were refrigerated again until use. Using pipets, 10 mL of each gasoline sample was transferred from the 60 mL bottle to a pre-cleaned 40-mL amber VOA glass vial with a plastic screw cap and a Teflonlined septum. The vial was capped immediately after sample introduction. Tedlar sample bags, each fitted with a QuickConnect connector and a port with a Teflon-lined septum, were filled with zero nitrogen to their full capacity (6 L) and evacuated. This procedure was repeated once. One liter of zero nitrogen was then added to each bag. All glass sample vials, Tedlar sample bags, and gastight syringes were placed inside a sealed housing (SHED) maintained at 38 °C (100 °F) for 2 h. Then, working inside the sealed housing and using gastight syringes, 0.3 mL of headspace vapor was removed from each vial and injected into the sample bags through the port with the Teflon-lined septum. At room temperature, each bag was filled by flowing 50 mL of zero nitrogen through the Teflonlined septum port and 4 L through the QuickConnect port. The bags were kept at room temperature for 2 h prior to GC analysis. Speciated hydrocarbon concentrations were measured in headspace vapor samples using GC/FID (12). C2 to C5 hydrocarbons were quantified using a 50 m × 0.32 mm i.d. alumina/potassium chloride PLOT column, with a 25 m ×
0.53 mm i.d. Carbowax precolumn. C6 to C12 hydrocarbons, ethanol, and MTBE were quantified using a DB-1 column, 60 m × 0.32 mm i.d., and 1 µm film thickness. Hydrocarbon concentrations were determined by integrating GC peak areas and using response factors determined from a NIST propane standard during calibration. To calculate the ethanol and MTBE concentrations, the corresponding experimentally determined GC response factors were applied: 0.685 for ethanol and 0.831 for MTBE (relative to propane ) 1.000). Further description of all of the above GC analytical procedures is provided elsewhere (10-12). Assessment of Reactivity. Carter’s maximum incremental reactivity (MIR) scale (13,14) was used together with organic compound speciation profiles to compute normalized reactivity R (g O3/g VOC emitted) for each fuel and headspace vapor sample:
R)
∑(MIR) w i
(1)
i
i
where wi is the weight fraction of species i present in the fuel or its headspace vapors. Prediction of Headspace Vapor Composition. Partial pressures pi of organic compounds present in gasoline headspace vapors were predicted using the following vaporliquid equilibrium relationship:
pi ) γixip°i
(2)
where γi is the liquid-phase activity coefficient of species i, xi is the mol fraction of species i in liquid fuel, and p°i is the vapor pressure of compound i in pure liquid form. Using predicted partial pressures of individual compounds and their sum, it is straightforward to compute vapor phase mol fraction fractions, and to convert mol fractions to weight fractions to obtain the final predicted headspace vapor composition profiles. Pure liquid vapor pressures were predicted as a function of temperature using the Wagner equation,
ln p°r )
aτ + bτ1.5 + cτ3 + dτ6 Tr
(3)
where p°r ) p°i/pc is reduced vapor pressure, Tr ) T/Tc is reduced temperature, pc and Tc are critical point pressure and temperature, and τ ) 1-Tr. Values of pc, Tc, a, b, c, and d are tabulated for numerous individual organic compounds in Appendix A of Reid et al. (15). Bennett et al. (16) used a dynamic Stage-Muller still to measure vapor pressure (P), temperature (T), and liquid and vapor phase mol fractions (x and y, respectively) of fivecomponent mixtures containing ethanol, an alkane (2,2,4trimethylpentane), an alkene (1-heptene), a cycloalkane (methylcyclohexane), and an aromatic (toluene). In the present study, activity coefficients γi were calculated from P, T, x, and y measurements reported in Table 3 of Bennett et al.:
γi )
Pyi
(4)
p°i(T)xi
Note, p°i(T) was predicted using the Wagner equation (eq 3), as shown in Table 1. Predicted vapor pressures for 2,2,4trimethylpentane and methylcyclohexane agree to within (0.2 kPa of measurements made by Wu et al. (17) at 25, 38, and 60 °C. For the purpose of estimating liquid-phase activity coefficients for real ethanol-gasoline blends, the system was modeled as a binary mixture with x1 ) ethanol mol fraction, VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1: Pure Compound Vapor Pressuresa of Ethanol and Selected Gasoline Hydrocarbons at 3 Temperatures vapor pressure (kPa)
ethanol 2,2,4-trimethylpentane methylcyclohexane 1-heptene toluene a
T ) 25 °C
T ) 38 °C
T ) 60 °C
7.9 6.6 6.2 7.5 3.8
16.1 11.9 11.2 13.6 7.2
47.0 28.7 27.0 33.1 18.5
Estimated using eq 3 and data from Appendix A of Reid et al. (15).
and x2 ) sum of all hydrocarbon mol fractions. It was assumed that the amount of ethanol present in solution was the dominant factor controlling activity coefficients for all compounds present in liquid fuel. Activity coefficients calculated for the four hydrocarbons listed above were assumed to apply to all other hydrocarbons present in gasoline. As discussed below, the hydrocarbons measured by Bennett et al. all had similar activity coefficients for any given ethanol mol fraction. In addition, the universal functional-group activity coefficent (UNIFAC) model (18) was used to provide independent estimates of liquid-phase activity coefficients. This model has been used previously to estimate vapor pressure and headspace vapor composition of gasoline containing ethanol (19). Activity coefficients γi were calculated for each species as follows:
ln γi ) ln γCi + ln γRι
(5)
where γCi is a combinatorial term that accounts for differences in molecular volume and surface area, and γRι is a residual term that accounts for interactions among functional groups that comprise the molecules present in solution. UNIFAC was used with 24 components (12 alkanes, 4 cycloalkanes, 1 alkene, 5 aromatics, ethanol, and MTBE). A small number of surrogate species was used to represent all of the alkenes, C7+ alkanes, C7+ cycloalkanes, and C9+ aromatics present in chemically complex gasoline mixtures. Greater resolution in the modeling of C4 and C5 alkenes may be warranted in other cases, but here all of the fuel samples had low (x < 0.005) total alkene mol fractions. A full listing of components, molecular subgroups, and liquid-phase mol fractions used in UNIFAC modeling is included in the Supporting Information that accompanies this paper. UNIFAC calculations were performed using a computer program provided by Sandler (20).
Results Measured liquid fuel composition profiles are presented in Figure 1 for each of the ethanol-containing gasoline samples (regular, mid-, and premium grades). Fuel oxygen content ranged from 1.2 wt % oxygen for the regular grade sample, to 2.8% for the mid-grade sample, and 3.4% for the premium grade sample. Almost all of the oxygen content was due to the presence of ethanol in the fuel samples; MTBE accounted for ∼4% of fuel oxygen content in the regular grade sample, and
